Recent studies show that auditory-nerve degeneration is associated with both normal aging and noise-induced temporary hearing loss. Though auditory-nerve degeneration in older listeners and individuals with a history of noise overexposure may contribute to perceptual difficulties in these groups, this hypothesis is currently unexplored. Individuals in these groups commonly report great difficulty understanding speech under real- world listening conditions. The proposed research will test the hypothesis that auditory-nerve degeneration causes adverse changes in central processing that ultimately degrade perception of complex signals in background noise. Parallel neurophysiological recordings from the auditory mid-brain and behavioral experiments involving complex stimuli in background noise will be conducted in the budgerigar because this avian species has human-like perceptual capabilities. Acoustic stimuli will include speech tokens and natural budgerigar calls. The K99 mentored research will use parallel neurophysiological and behavioral approaches to identify functional cell classes in the mid-brain that contribute to robust perceptual abilities in normal-hearing animals. Having advanced our knowledge of normal-hearing perception, the R00 independent research will quantify changes in mid-brain coding and perception in noise with auditory-nerve degeneration. Auditory-nerve degeneration will be induced with kainic acid based on previous results showing that this agent selectively damages nerve fibers while leaving cochlear hair cells intact. The results of this research are expected to have high impact because they will quantify, possibly for the first time, the perceptual consequences of auditory-nerve degeneration under real-world listening conditions. Furthermore, the parallel physiological and behavioral data provided by this work are expected to provide unique insight into the physiological changes underlying perceptual difficulties. Ultimately, this knowledge can help guide new technologies and treatment strategies aimed at restoring performance in people with speech perception problems in noise. The K99 mentored phase of the proposed research will aid in the candidate's career development by expanding his previous neurophysiological training in auditory-nerve fiber recordings into the central auditory system. Furthermore, the candidate will gain valuable new skills in animal psychoacoustics. The University of Rochester is an excellent environment for the K99 research because this institution has an active systems neuroscience community including the Sponsor and other key faculty. The new combination of skills provided by this work will allow the candidate to achieve his goal of using parallel physiological and behavioral approaches to help pinpoint the physiological changes underlying human communication problems.